1. Field of the Invention
The present invention relates to a method for manufacturing a fluorescent lamp. In particular, the present invention relates to a method for cutting a glass tube used for a fluorescent lamp.
2. Description of the Related Art
In the last few years, the consciousness for saving energy has increased, and as an alternative energy-saving light source that takes the place of low efficiency bulbs, the use of compact fluorescent lamps has been promoted.
For example, JP4(1992)-58137B2A proposes a typical structure of an arc tube and a method for manufacturing the same. Within this manufacturing method, the steps of processing a glass tube are shown in FIG. 7A to FIG. 7C as follows:
(A) A glass tube 30 coated with a fluorescent substance 31 on the inner surface is rotated around a tube axis, and while the glass tube is rotated, a glass wall at the area near the center of the glass tube 30 is heated by a burner up to a temperature that is slightly higher than the softening temperature of the glass tube.
(B) End portions of the glass tube that are not softened are drawn away from each other, and the cutting portion cut is thinned by the heat of the burner.
(C) The glass tube 30 is cut into a pair of glass tubes 32 and 33. At this stage, the pair of glass tubes 32, 33 have self-closing end walls 34 and 35 that are formed at the cut ends. In order to prevent cracks from occurring in the glass tubes caused by micro leakage or glass distortion in the end walls 34, 35, air is blown into the softened end walls 34, 35 while they are placed against an abutment. In this way, the end walls are processed to be substantially perpendicular to the tube axis direction of the glass tubes 32, 33 and also into flat surfaces. As a result, dislocation portions (edge portions) between the end walls 34, 35 and the glass tubes 32, 33 are processed to be 0.4 to 0.8 times as thick as the average thickness of the glass tubes 32, 33. Furthermore, the central portions of the end walls 34, 35 are processed to be 1 to 1.5 times as thick as the average thickness of the glass tubes 32, 33.
Next, as illustrated in FIG. 8, when an arc tube 36 for a lamp is manufactured, glass stems 39 and 40 that respectively hold electrode filament coils 37 and 38 are sealed at other end portions of the pair of the glass tubes 32, 33 that are open. The glass tubes 32, 33 are interconnected through a junction bridge 41 in the vicinity of the end portions that are cut and self-closed so as to form a line of discharge path inside the arc tube 36. Rare gas such as mercury and argon is sealed in the arc tube 36. In the completed form, the arc tube 36 of the lamp is equipped with a base 42 that is attached to the end portion on the electrode side.
Furthermore, JP11(1999)-40057A proposes another improved method for further suppressing cracks and leakage around the respective end walls 34, 35 of the glass tubes 32, 33 that are cut and self-closed as shown in FIG. 7C. The problem with the case illustrated in FIG. 7C is that the glass residual distortion exists in the glass due to uneven thicknesses of the end walls, and that cracks and leakage occur around the end walls 34, 35 when the arc tube is heated in the following lamp manufacturing process as well as over the lighting life of the lamp, or when shocks or vibrations are inflicted on the lamps during transportation. Therefore, as illustrated in FIG. 9A to FIG. 9C, a glass tube that rotates around a tube axis is heated at the area around the center by a burner, and the glass tube is cut into two pieces that make a pair having end walls that close by itself. Then, while end walls 47 and 48 of self-closing glass tubes 45 and 46 are still soft, a gas (air or nitrogen) is supplied into the glass tubes 45, 46 respectively from the other ends that are open, so that the end walls 47, 48 respectively are inflated to form convex surfaces. In this way, the glass tubes are processed to have the end walls with uniform thicknesses and also without glass residual distortion.
The performance characteristics of the above compact fluorescent lamp interconnected with a junction bridge are that when the lamp is lit up, the coldest points that control the vapor pressure of mercury in the tubes to be within the optimal range are formed in the end walls 34, 35 of the glass tubes 32, 33 that are cut and self-closed as in the example shown in FIG. 7C. Furthermore, the luminous flux of the lamp is maintained around the maximum value.
By using the compact fluorescent lamps manufactured according to the conventional techniques described above in which the glass tube is cut and closed in the glass processing step and then interconnected by a bridge, defective percentage of the lamps during the manufacturing process and also on the market where the lamps actually are consigned was examined and analyzed over a long period of time. As a result, for example, with regard to the compact fluorescent lamps of 36W illustrated in FIG. 8 in which two glass tubes having an outer diameter of 20 mm are joined, the defective percentage of the lamps manufactured according to the method described in JP4(1992)-58137A above was 0.5% in the manufacturing process due to cracks, leakage, or damages around the end walls of the glass tubes that are cut and closed. On the other hand, the defective percentage of the lamps manufactured according to the method described in JP11(1999)-40057A was 0.02%. As far as the defective percentage in the manufacturing process of the lamps is concerned, the latter method for processing the glass is effective in suppressing cracks and leakage.
However, looking at the defective percentage of the lamps on the market, even with the lamps manufactured according to the method of JP11(1999)-40057A, the defective percentage of the lamps due to cracks, leakage, or damages around the end walls of the glass tubes that are cut and closed proved to be still as comparatively high as about 0.1%. The lamp defects mostly were glass damages around the end walls 47, 48 described above, which can be concluded to be the result of mechanical shocks particularly inflicted during the transportation and the attachment of the lamps. Specifically, even though the end walls 47, 48 of the glass tubes 32, 33 that are cut and closed according to the method of JP11(1999)-40057A are improved also in regard to shock resistance compared to those manufactured according to the glass processing step of JP4(1992)-58137A, the glass thicknesses are reduced by about 27% to 43% versus those of the original glass tube walls. This fact basically is considered to deteriorate the mechanical strength of the end walls 47, 48 and to cause the comparatively high defective percentage of the lamps on the market. Therefore, to reduce the defective percentage of such compact fluorescent lamps during the manufacturing process and also on the actual market, it is important to improve the mechanical strength of the end walls 47, 48 of the glass tubes that are cut and self-closed.
In order to solve the conventional problems described above, it is an object of the present invention to provide a method for manufacturing a fluorescent lamp in which end walls of glass tubes that are cut and self-closed have improved mechanical strength and the defective percentage of lamps is reduced even more during the manufacturing process and on the market than that of the conventional lamps.
To achieve the above objects, the method for manufacturing a fluorescent lamp of the present invention is a method for manufacturing a glass tube used for a fluorescent lamp having an arc tube constructed by joining a plurality of glass tubes. According to this method, a straight glass tube held at both ends is rotated, and a predetermined cutting portion of the glass tube is softened by heating, and then the glass tube is moved in the heated state in the direction toward the predetermined cutting portion to form a glass accumulated portion at the area of the predetermined cutting portion. Next, the glass tube is drawn away in the direction receding from the predetermined cutting portion, and the predetermined cutting portion of the glass tube is cut into two glass tubes having self-closing end walls.
According to this method, the end walls of the glass tubes that are cut and self-closed can be formed thicker and more uniform compared to those obtained by the conventional techniques, and the mechanical strength of the end walls also can be improved. As a result, the defective percentage of the lamps during the manufacturing process and also on the market due to cracks, leakage, or damages in the end walls of the glass tubes can be reduced.